Thermosetting powder coating compositions are well known in the art and are widely used as coatings for electric appliances, bicycles, garden furniture, accessories for the automotive industry, general metal parts and the like. Thermosetting powders consist of a mixture of a primary resin and one or more cross-linkers, often called hardeners or curing agents. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. In so far as possible, each particle contains all of the ingredients in the formulation. The powder is applied to the substrate, usually but not limited to a metal, and fused to a continuous film by baking.
A problem with powder coating compositions is that they frequently have low glass transition temperatures (T.sub.g) which will lead to agglomeration or sintering of the powdered coating particles when they are stored at elevated temperatures for a prolonged duration of time. The agglomerated powder coating can cause problems during application and may require re-milling, which itself can cause further problems by, for example, adversely affecting the particle size distribution of the re-milled powder coating. Often, powder coating compositions which are formulated to avoid problems with agglomeration and sintering do not provide optimal coatings properties.
It is an object of the invention to provide a powder coating composition which has a relatively high glass transition temperature, will resist agglomeration during storage and which composition will cure without a catalyst at a relatively low temperature.
It is an object of the invention to provide a powdered coating composition which will maximize film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss, yet also provide a coating composition with a relatively high glass transition temperature and which will cure without a catalyst at relatively low temperatures.
It is another object of the invention to provide a powdered coating composition which can be cured at temperatures as low as about 150.degree. C. without the use of an effective amount of catalyst.
It is another object of the invention to provide a powder coating composition with a desirable melt viscosity.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.